Helmets have been recently worn in various types of fields. For example, a worker who works in various environments, such as a building site, a logging site, a factory, a plane operation, facilities, vehicle driving, and the like, is to indispensably wear a helmet. In addition, as hobbies become diverse like skis, inline skates, a board, a bicycle, a motorcycle, an amusement park, and the like, cases where ordinary people are to wear helmets occur frequently.
Helmets are demanded in various environments as described above. However, if a helmet is used, the helmet may cover ears, and if ambient noise is very serious, generally output audio may not be frequently appropriately heard. Also, if the helmet is used, there are many cases where both hands may not be quite freely used.
For solving this problem, an audio apparatus which is attached to a helmet and provides audio through a vibration has been provide. However, a sound quality or a sound volume of audio which is proved through a vibration becomes different according to various main factors such as a type, a thickness, and the like of a helmet. Therefore, it is difficult to provide a user with an optimum sound quality.